Where I work we get celebrities through the building all the time. We have had Presidents, the Dalai Lama, various Congress people and Senators, many movie stars, The Beltway Batman, famous artists and musicians, athletes, you name it. If you run into them or happen to be riding the same elevator or something you just act the same as you would toward anyone else here.

Say hi back, if a little stiffly. It's not unusual for people to greet strangers they pass on the street by saying things like "Good morning" and so on so I would assume it was that sort of interaction. Either that or they're about to ask me for directions or where to find parking.